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From: Dave on 17 Apr 2008 18:15 On Apr 17, 5:13 pm, "Steve" <sjbur...(a)comcast.net> wrote: > "Joerg" <notthisjoerg...(a)removethispacbell.net> wrote in message > > news:U5MNj.6956$GE1.6193(a)nlpi061.nbdc.sbc.com... > > > > > qrk wrote: > >> On Thu, 17 Apr 2008 09:43:09 -0700 (PDT), Dave <dhsch...(a)gmail.com> > >> wrote: > > >>> Does anybody out there have a good methodology for determining your > >>> optimal FPGA pinouts, for making PCB layouts nice, pretty, and clean? > >>> The brute force method is fairly maddening. I'd be curious to hear if > >>> anybody has any 'tricks of the trade' here. > > >>> Also, just out of curiosity, how many of you do your own PCB layout, > >>> versus farming it out? It would certainly save us a lot of money to > >>> buy the tools and do it ourselves, but it seems like laying out a > >>> board out well requires quite a bit of experience, especially a 6-8 > >>> layer board with high pin count FPGA's. > > >>> We're just setting up a hardware shop here, and although I've been > >>> doing FPGA and board schematics design for a while, it's always been > >>> at a larger company with resources to farm the layout out, and we > >>> never did anything high-speed to really worry about the board layout > >>> too much. Thanks in advance for your opinions. > > >>> Dave > > >> Sure wish there was a slick way of doing FPGA pinouts. I usually use > >> graph paper and figure out the FPGA pinout to other parts to minimize > >> routing snarls. > > >> I do pcb layouts on my own and other folks designs. Our boards have > >> high-speed routing, switching power supplies, and high-gain analog > >> stuff; sometimes all on the same board. Unless the service bureau has > >> someone who understands how to lay out such circuitry and place > >> sensitive analog stuff near digital junk, it is more trouble to farm > >> out than do it yourself if you want the board to work on the first > >> cut. > > > Or find a good layouter and develop a long-term business relationship. My > > layouter knows just from looking at a schematic which areas are critical. > > He's a lot older than I am and that is probably one of the reasons why his > > stuff works without much assistance from me. Nothing can replace a few > > decades of experience. > > >> Doing your own layout will take a lot of learning to master the PCB > >> layout program and what your board vendor can handle. It will take 5 > >> to 10 complicated boards to become mildly proficient at layout. I > >> don't know about saving cost. Your time may be better spent doing > >> other activities rather than learning about layout and doing the > >> layouts. ... > > > Yep, that's why I usually do not do my own layouts. Occassionally I route > > a small portion of a circuit and send that to my layouter. No DRC or > > anything, just to show him how I'd like it done. > > >> ... The upside to doing your own layout - you control the whole > >> design from start to finish. If you have a challenging layout, you'll > >> have a much higher probability of having a working board on the first > >> try which has hidden savings (getting to market earlier <- less > >> troubleshooting + less respins). > > >> --- > >> Mark > > > -- > > Regards, Joerg > > >http://www.analogconsultants.com/ > > > "gmail" domain blocked because of excessive spam. > > Use another domain or send PM. > > I agree with Joerg. Good high speed or mixed signal PCB layout is a career > choice, and we electrical engineers already chose our career. A good layout > requires someone who understands not just the software package, but the > details of how the manufacturing operation is going to proceed, what the > limits of the processes are, what the assembly operations require of the > board, and is anal about things like footprint libraries and solder mask > clearances and a thousand other details that I'm only partially aware of. > The more complex your design, the more critical these things become. > > I have two good local outfits for farming out boards. For complex stuff, > they know I'll come to their place and sit next to the designer for a good > bit of the initial placement. While we are doing placement, we are also > discussing critical nets, routing paths, layer usage, etc. That gives us > direct face to face communication and avoids spending lots of time trying to > write/draw everything in gory detail (which gets ignored or misunderstood a > lot of the time). That investment pays big dividends in schedule and board > performance. > > Don't be fooled by the relatively low cost of the software. That's not where > the big costs are. > > I once laid off my entire PCB layout department and sent all the work > outside, because although my employees all knew how to use the software, > none of them could tell me what their completion date would be, or how many > hours it would take, and they certainly weren't interested in meeting > schedules. The outside sources would commit to a cost and a delivery date. > And we already knew they could meet our performance objectives. Fixed price > contracts are great motivators. Missing an engineering test window, or > slipping a production schedule because of a layout delay can be enormously > expensive. > > Of course, if I had let my engineers do their own layouts, the motivation > would have been present, but the technical proficiency would not. How > proficient can anyone become if they only do layout a few times a year? > Also, on many projects engineers use the layout period for other important > things like documentation, test procedures, writing test code, etc. Doing > your own layout serializes these tasks and will stretch your schedule. > > So my advice is to keep doing what you have been doing. Its far more likely > that its the cheapest approach, even though you occasionally have to write a > big check. > > Steve I tend to agree with the 'farm-it-out' crowd. Unfortunately, my current employer doesn't want to work with my previous layout people, so I've been trying to search for a new partner. I've found plenty of board fab and assembly places, but not so much on the layout. It made me think that the rest of the world did their own layout. The opinions look pretty split from the replies here, maybe it comes down to how many times you do a layout each year, and how much you enjoy that sort of work. I definitely think it's something you have to do fairly often to keep your chops up. Andy, I'd also like to hear more about your pin-swap FPGA design flow - what tools do that? Also curious about any timing issues that have been caught after the pin-swap. Thank you all very much for the info. If any of you find yourself in the Baltimore area, I owe you a crabcake sandwich and a beer. Dave
From: Jeff Cunningham on 17 Apr 2008 18:51 Dave wrote: > > I tend to agree with the 'farm-it-out' crowd. Unfortunately, my > current employer doesn't want to work with my previous layout people, > so I've been trying to search for a new partner. I've found plenty of > board fab and assembly places, but not so much on the layout. It made Some of the PCB software vendors have lists on their web site of independent consultants and layout houses that use their software. I went on the Mentor site and found zillions of layout people. -Jeff
From: Joel Koltner on 17 Apr 2008 19:20 "Dave" <dhschetz(a)gmail.com> wrote in message news:dfc27fc0-c07f-456c-80cb-b31e867dd253(a)d1g2000hsg.googlegroups.com... > The opinions > look pretty split from the replies here, maybe it comes down to how > many times you do a layout each year, and how much you enjoy that sort > of work. I definitely think it's something you have to do fairly often > to keep your chops up. I think what you're seeing is that fact that, by sheer volume of products, guys doing relatively low-speed digital stuff completely dominate those doing very low-level analog, RF, microwave, or truly high-speed digital. In the former case, it just doesn't really matter that much how you layout the board. Sure, there are definitely better ways and worse ways, but even up to clock rates pushing 100MHz, for digital stuff I think you can give a guy about an hour of education and he'll be able to make boards work just fine. Another point to keep in mind is that there's a significant difference between being able to design a board well when you're talking relatively small volume production for high-end commercial or military customers where you can afford to just toss in some extra layers and pay for blind or buried vias or tigether tolerances if you're at all unsure of how well your layout skills really are vs. designing a complex board for highly cost-competitive mass-markets. The later requires a lot of skills that are anything but what is commonly taught! (E.g., typically at tech seminars you'll hear people preaching, "throw in a ground plane!" -- an action that saves many an otherwise broken design, but one which might not be possible if your competition has already figured out how to live without one.) I'm a big advocate of giving "technical interviews" to would-be PCB layout guys based on what your needs are. If you're doing, e.g., RF or high-speed digital design, ask them how line impedances change with changes in board and trace dimensions, what near-end and far-end crosstalk look like on a scope, what they think about splitting up ground planes, how they'd route some simple circuits, etc... Usually you can find out pretty quickly what their skills are whether or not they're adequate or if they'd need a bit more hand-holding... which could be fine too, if you have the time and the price is right. > Andy, I'd also like to hear more about your pin-swap FPGA design flow > - what tools do that? It's a common feature in most PCB tools to allow pin (and gate) swapping based on the component's library entry being set up to designate which pins and gates are "swappable." After doing so, most of them will produce a simple ASCII "was-is" text file that list the old pin name and the new one, which can be imported back into a schematic capture program or used to update your FPGA place & route constraints. (PADS will do all this, where Pulsonix unfortunately does pin & gate swapping quite nicely but will only update a Pulsonix schematic "directly" rather than providing you with the option to generate a was-is file.) ---Joel
From: Joerg on 17 Apr 2008 19:46 Joel Koltner wrote: > "Dave" <dhschetz(a)gmail.com> wrote in message > news:dfc27fc0-c07f-456c-80cb-b31e867dd253(a)d1g2000hsg.googlegroups.com... >> The opinions >> look pretty split from the replies here, maybe it comes down to how >> many times you do a layout each year, and how much you enjoy that sort >> of work. I definitely think it's something you have to do fairly often >> to keep your chops up. > > I think what you're seeing is that fact that, by sheer volume of products, > guys doing relatively low-speed digital stuff completely dominate those doing > very low-level analog, RF, microwave, or truly high-speed digital. In the > former case, it just doesn't really matter that much how you layout the board. > Sure, there are definitely better ways and worse ways, but even up to clock > rates pushing 100MHz, for digital stuff I think you can give a guy about an > hour of education and he'll be able to make boards work just fine. > Not anymore. Part of my daily bread is earned salvaging designs where someone thought "Oh, it's just slow stuff". But it ain't grampa's old SN7400 anymore, today's logic chips are fast. Some like the tiny logic chips swing their outputs within very few nanoseconds. Then some unexpected weirdnesses show up. Everyone thinks it's software but in reality crosstalk has manifested itself. Other times the moment of truth cometh at the EMC lab when a thick forrest shows up on the spectrum analyzer. > Another point to keep in mind is that there's a significant difference between > being able to design a board well when you're talking relatively small volume > production for high-end commercial or military customers where you can afford > to just toss in some extra layers and pay for blind or buried vias or tigether > tolerances if you're at all unsure of how well your layout skills really are > vs. designing a complex board for highly cost-competitive mass-markets. The > later requires a lot of skills that are anything but what is commonly taught! > (E.g., typically at tech seminars you'll hear people preaching, "throw in a > ground plane!" -- an action that saves many an otherwise broken design, but > one which might not be possible if your competition has already figured out > how to live without one.) > And don't split that plane. But yes, often one has to make do with two-layer phenolic. That is often true art. BTW does that little switcher work? [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joel Koltner on 17 Apr 2008 20:52
"Joerg" <notthisjoergsch(a)removethispacbell.net> wrote in message news:ZjRNj.9697$2g1.9469(a)nlpi068.nbdc.sbc.com... > Not anymore. Part of my daily bread is earned salvaging designs where > someone thought "Oh, it's just slow stuff". But it ain't grampa's old SN7400 > anymore, today's logic chips are fast. OK, ok, good point. Doesn't someone now have a logic family that's purposely been slowed down due to this "problem?" > And don't split that plane. But yes, often one has to make do with two-layer > phenolic. That is often true art. From Thomas Lee (Stanford) in "Planar Microwave Engineering": In extremely low-cost consumer devices (e.g., toys, pocket radios, etc.), an even less expensive board material is not infrequently encountered. Phenolic is often a caramel brown, typically has an "organical chemical" odor, and is remarkably lossy. Although phenolic is occasionally used for RF toys up to 100MHz, it is totally insuitable for serious applications. It is mentioned here simply to answer the question: "What is that cheap, malodorous board made of?" :-) I know, I know, he's living in an ivory tower a bit, but he is one smart cookie. > BTW does that little switcher work? I've had that board back for about a week, although I haven't actually tested out the switcher yet since the DSP guy isn't interested in working with the new (digital) board until the new RF board comes back (and gets tested) as well, which is still a couple weeks out. (There's this "Big Tester Board" that's needed to test the RF board and said BTB has spent something over a week bouncing around engineering getting tweaked/fixed/etc... we'll be paying a premium to actually get it fabbed in time to start testing RF boards at this point, unfortunately :-( .) I can and probably should just put a dummy load on the switcher, turn it on, and see if there's any obvious problems before the DSP guy starts looking at his clock jitter. Tomorrow sounds like a good day for that... ---Joel |